Set width for spacer
<?xml version="1.0" encoding="utf-8"?> <mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute"> <mx:HBox> <mx:Button label="First"/> <mx:Spacer width="40"/> <mx:Button label="Second"/> <mx:Button label="Third"/> </mx:HBox> </mx:Application>